Method for manufacturing a monolithic LED micro-display on an active matrix panel using flip-chip technology and display apparatus having the monolithic LED micro-display

ABSTRACT

A high-resolution, Active Matrix (AM) programmed monolithic Light Emitting Diode (LED) micro-array is fabricated using flip-chip technology. The fabrication process includes fabrications of an LED micro-array and an AM panel, and combining the resulting LED micro-array and AM panel using the flip-chip technology. The LED micro-array is grown and fabricated on a sapphire substrate and the AM panel can be fabricated using CMOS process. LED pixels in a same row share a common N-bus line that is connected to the ground of AM panel while p-electrodes of the LED pixels are electrically separated such that each p-electrode is independently connected to an output of drive circuits mounted on the AM panel. The LED micro-array is flip-chip bonded to the AM panel so that the AM panel controls the LED pixels individually and the LED pixels exhibit excellent emission uniformity. According to this constitution, incompatibility between the LED process and the CMOS process can be eliminated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present subject matter relates generally to manufacturing a LightEmitting Diode (LED) micro-display on an Active Matrix (AM) panel, andmore specifically to manufacturing an LED micro-display on an AM panelusing flip-chip technology.

2. Related Art

LED array displays made with individually packaged devices have beenwidely used for various applications. In recent years, differenttechniques have been exploited to fabricate monolithic,passively-addressable LED arrays. The array dimensions and pixelbrightness in conventional passively-addressable LED arrays were limitedby the loading effect in the same row or column. Thus, various newaddress schemes and fabrication technologies are suggested to improvethe operating effectiveness of monolithic LED arrays.

For example, U.S. Pat. Nos. 5,789,766, 5,827,753, and 5,893,721 relateto methods of fabricating an LED array and driving circuitry thatincludes sequentially forming overlying layers of material on thesurface of a semiconductor substrate, the layers cooperating to emitlight when activated. The insulating layer is formed on the layers andthe layers are isolated into an array area and the driver circuitry areawith row and column drivers dividing the array area into an array ofLEDs arranged in rows and columns. Row and column driver circuits areformed on the insulating layer in the driver circuitry area. Row busesindividually couple each LED in the array to corresponding drivercircuits. But the circuitry layer formed on the insulating layer canonly be a thin film device like a-Si TFT or poly-Si TFT, however, thesehave low field effect mobility and cannot provide enough current for theLEDs. On the other hand, these three patents adopt a bottom-emittingconfiguration and the aperture of the array is limited by the circuitryarea, thereby resulting in a relatively low light efficiency.

U.S. Patent Application Publication No. 2008/0194054 relates to a methodof fabricating an LED array package structure having a siliconsubstrate. The LED array package structure includes a silicon substratehaving a plurality of cup-structures thereon, a reflective layerdisposed on the silicon substrate, a transparent insulation layerdisposed on the reflective layer, a conductive layer disposed on thetransparent insulation layer and a plurality of LEDs disposedrespectively on the conductive layer in each cup-structure. In this way,the LED array can only operate in a passive mode and each LED size is aslarge as tens millimeters. Hence, the LEDs made by this method oftensuffer from bad illumination uniformity and the low resolution.

U.S. Pat. No. 6,975,293 relates to five types of driving circuits for anactive matrix LED display. The circuits are composed of four MOStransistors, each of which has a drain and a source. An anode of an LEDis coupled to a source of a driving transistor and a cathode of the LEDis coupled to a second voltage. These methods are suitable for an OLEDarray display, but not suitable for an LED micro-display array becausethe LED process is not compatible with the CMOS process.

U.S. Patent Application Publication No. 2008/0171141 relates to methodsof fabricating LED array structures including multiple vertical LEDstacks coupled to a single metal substrate. Such an LED array may offerbetter heat conduction and an improved matching of LED characteristics(e.g., forward voltage and emission wave length) between the individualLED stacks compared to conventional LED arrays. But the LED array inthis reference can only act as a single LED which can only light on andoff together and cannot control the LED pixel individually andprecisely.

SUMMARY OF THE INVENTION

The present subject matter provides a method for manufacturing amonolithic LED micro-display on an active matrix panel using flip-chiptechnology and a display apparatus having the monolithic LEDmicro-display.

According to an aspect, the subject matter is directed to a method formanufacturing a monolithic Light Emitting Diode (LED) micro-displaypanel on an Active Matrix (AM) panel, comprising: providing a substrateof the LED micro-display panel; providing a plurality of overlayinglayers of material on a surface of the substrate, the plurality ofoverlaying layers of material being configured in combination to emitlight when activated; patterning the plurality of overlaying layers ofmaterial by removing a part thereof all the way down to the surface ofthe substrate; depositing a current spreading layer on the plurality ofoverlaying layers of material and the surface of the substrate;providing a metal multilayer on the patterned plurality of overlayinglayers of material and the surface of the substrate; patterning themetal multilayer in such a configuration that a first portion of themetal multilayer, which lies on the patterned plurality of overlayinglayers of material, and a second portion of the metal multilayer, whichlies on the surface of the substrate are conductively disconnected,thereby forming an micro-array of multiple monolithic LEDs; andcombining the monolithic LED micro-display panel with the AM panel,which includes a plurality of active control circuit chips fixedthereon, the plurality of active control circuit chips being providedwith conductive solder material thereon, in such a configuration thatthe monolithic LEDs are flip-chip bonded to the active control circuitchips via the conductive solder material and each of the monolithic LEDsis electrically insulated from one another, whereby each of themonolithic LEDs is independently controllable by corresponding one ofthe active control circuit chips bonded thereto.

According to another aspect, the subject matter is directed to a methodfor manufacturing an assembly of a monolithic Light Emitting Diode (LED)micro-display panel including a plurality of LEDs thereon and an ActiveMatrix (AM) panel, comprising: providing a substrate of the AM panel;providing active control circuit chips on the substrate of the AM panel;providing conductive solder material on the active control circuitchips; and combining the AM panel with the monolithic LED micro-displaypanel in such a configuration that the active control circuit chips areflip-chip bonded to the plurality of monolithic LEDs via the conductivesolder material.

According to a further aspect, the subject matter is directed to a LightEmitting Diode (LED) display comprising: a LED panel mounted with aplurality of LEDs arranged in rows and columns; and an Active Matrix(AM) panel mounted with a plurality of active control circuits, whereinthe LED panel is combined with the AM panel in such a configuration thateach of the plurality of LEDs is associated with each of the activecontrol circuits, each pair of an LED and an associated active controlcircuit being electrically insulated from other pairs of LEDs andassociated active control circuits in the LED display, each LED beingindependently controllable by each associated active control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the subject matter will be apparent withreference to the examples in the following description and withreference to the accompanying drawings, wherein

FIG. 1 is a simplified prior art schematic diagram of a top plan view ofa passive LED matrix wherein the passive LED matrix includes a pluralityof LEDs arranged in rows and columns with anodes in a same columnconnected to one another via column buses and cathodes in a same columnconnected to one another via row buses.

FIG. 2 shows a top plan view of the prior art passive LED matrix, asschematically shown in FIG. 1.

FIG. 3 schematically shows a manufacture of an AM LED display accordingto the present subject matter by conductive bonding an LED micro-arraypanel with an AM panel.

FIG. 4 schematically shows a layout of an 8×8 LED micro-array accordingto the present subject matter wherein individual LEDs on a same columnare connected to one another by their n-electrodes at the end of eachrow as cathodes while individual LEDs on a same row are connected to anoutput of an AM panel through the solder bumps as anodes.

FIG. 5 shows a turn-on voltage distribution of eight LEDs in the samerow according to the present subject matter.

FIG. 6 schematically shows a configuration of driving circuitry on an AMpanel according to the present subject matter.

FIG. 7 shows a cross-sectional diagram of two neighboring LED pixels inan LED micro-array as shown in FIG. 4.

FIG. 8 shows a configuration of 8×8 LED driving circuits on an AM panelaccording to the present subject matter.

FIG. 9 is a schematic diagram of a pixel driving circuit according tothe present subject matter.

FIG. 10 shows a microscopic image of an assembled AM LED micro-arraypanel.

FIG. 11 shows a typical current-voltage (I-V) characteristic of anindividual pixel in an AM LED micro-array panel.

FIG. 12A and FIG. 12B respectively show fully turned-on and individuallyturned-on images of an AM LED micro-array panel.

FIG. 13 shows solder bumps on a top of an AM panel.

DETAILED DESCRIPTION OF THE INVENTION

The Figures are diagrammatic and not drawn to scale. In the Figures,elements which correspond to elements already described have the samereference numerals.

FIG. 1 is a simplified schematic diagram of a top plan view of a priorart passive LED matrix 100 wherein the passive LED matrix 100 includes aplurality of LEDs 101 arranged in rows and columns with anodes in a samecolumn connected to one another via column buses 102 and cathodes in asame column connected to one another via row buses 103. FIG. 2 shows atop plan view of the prior art passive LED matrix, as schematicallyshown in FIG. 1.

FIG. 3 schematically shows a manufacture of an AM LED display 104according to the present subject matter by flip-chip boding an LEDmicro-array panel 105 with an AM panel 106. The flip-chip bonding is amethod for interconnecting semiconductor devices, such as IntegratedCircuit (IC) chips and micro-electromechanical systems, to externalcircuitry with solder bumps that have been deposited onto the chip pads.The solder bumps are deposited on the chip pads on the top side of thewafer during the final wafer processing step. In order to mount the chipto external circuitry, it is flipped over so that its top side facesdown, and aligned so that its pads align with matching pads on theexternal circuit, and then the solder is flowed to complete theinterconnect. In this embodiment, a plurality of active control circuitsmounted on the AM panel 104 are flipped over so that their top sidesface down, and aligned and coherently adhered to bonding pads ofcorresponding LEDs mounted on the LED micro-array panel 105. Thisprocedure will be described in more detail below with reference to FIG.5.

FIG. 4 schematically shows a layout of an 8×8 LED micro-array 105according to the present subject matter wherein individual LEDs 101(emission wavelength 440 nm) on a same column are connected to oneanother by their n-electrodes (not shown) via a bus 102 at the end ofeach row as cathodes 116 while individual LEDs 101 on a same row areconnected to an output of an AM panel through the solder bumps as anodes117. In this constitution, the current passes through an n-GaN layer andan n-metal bus line to reach the n-electrodes. The LED micro-array hassimilar electrical properties as a commercial 8×8 LED dot array. Asshown by commercial discrete power LED manufacturing, flip-chiptechnology can improve heat dissipation, reliability, andmanufacturability. A silicon substrate has a larger thermal conductivity(150 W/m·K) than a sapphire substrate (46 W/m·K), and well-developedflip-chip technology has been used with silicon for decades. In theepi-down (bottom-emitting) configuration, the p-electrode itself can bemade reflective, thus eliminating any absorption of the currentspreading layer and metal pads. In this way, the light output power andefficiency will be improved.

FIG. 5 shows a turn-on voltage distribution of eight LEDs in the samerow according to the present subject matter. The turn-on voltages ofLEDs, under the same 20 mA current injection, were strongly dependent onthe distance between each LED and n-electrode. Series resistance of thebus bars to the n-GaN contact strip resulted in increased turn-onvoltage with longer distance of dies from the contacts at the end ofeach column. For high-performance micro-displays, the variation ofturn-on voltage might cause a different junction temperature and/or acompensation of piezoelectric field between the individual LED pixels,and hence, a variation in lifetime and emitting wavelength of theindividual LEDs. The wavelength variation across the LEDs would resultin a poor angular homogeneity of color purity. In this embodiment, theturn-on voltage uniformity was greatly improved in a design with 40μm-wide one-side n-metal bus lines on each row. The turn-on voltagesvaried only from 3.30 to 3.70 V over the whole row under the samecurrent injection, as shown in FIG. 5.

FIG. 6 schematically shows a configuration of driving circuitry on an AMpanel according to the present subject matter. Transistor T1 serves as aswitching transistor and T2 serves as a driving transistor. When T1 isswitched on by a scan signal, a data signal switches T2 on and is storedin the capacitor C1. Then, T2 provides current light up the LED pixelwhose p-electrode is connected to the drain of T2. Driving transistor T2is designed with a large W/L ratio to warrant enough output current forthe LED pixel.

FIG. 7 shows a cross-sectional diagram of two neighbored LED pixels 107shown in FIG. 4 in an LED micro-array according to the present subjectmatter. An n-GaN layer 109, a Multiple Quantum Well (MQW) 110, and ap-GaN layer 111 were grown on a substrate 108. Silicon dioxide (SiO₂)masks were used for inductively coupled plasma (ICP) etching. The LEDwafer was etched all the way down to the substrate. Rows of themicro-array were defined and isolated in this step. A Plasma-EnhancedChemical Vapor Deposition (PECVD) SiO₂ mask and an ICP were used againto define the mesa structure of each LED pixel. A thin Ni/Au (5/5 nm)current spreading layer 112 was deposited onto the p-GaN layer 111 toform p-electrodes. Annealing in the atmospheric ambient at 570° C. for 5minutes was performed. Then, a metal layer 113 was evaporated to formn-electrodes and a reflective layer on the p-electrodes simultaneously.Finally, Silicon dioxide passivation 114 was applied onto the wafer.Openings in the SiO₂ layers were defined for flip-chip bonding. Thefabrication of an AM LED micro-array display will be described ingreater detail below.

First, fabrication of an LED micro-array panel is being described. Astandard Multiple Quantum Well (MQW) blue LED wafer (emission wavelength440 nm) grown on a sapphire substrate was used for fabrication of an LEDmicro-array. In place of the sapphire substrate, GaAs, SiC,Semi-insulating GaAs, or Quartz substrate can be used. Plasma EnhancedChemical Vapor Deposition (PECVD) grown SiO₂ masks were used for ICPetching. The LED wafer was etched all the way down to the sapphiresubstrate. Rows of the array were defined and isolated in this step. ThePECVD SiO₂ mask and ICP were again used to define the mesa structure ofeach LED pixel, with individual device size of 300×300 μm². A thin Ni/Au(5/5 nm) current spreading layer was deposited onto a p-GaN surface byelectron beam evaporation to form p-electrodes. Annealing at 570° C. inambient atmosphere for 5 minutes was performed. In place of the thinNi/Au current spreading layer, a thin Ag/ITO current spreading layer canbe used. Then, a Ti/Al/Ti/Au (30/120/10/30 nm) multilayer metal wasevaporated to form n-electrodes and a reflective layer on thep-electrodes simultaneously. Finally, SiO₂ passivation (or SiN_(x) orphotoresist) was applied onto the wafer. Openings in the SiO₂ weredefined, and a Ni/Au (500/30 nm) contact pad was formed in the openingfor flip-chip bonding.

Secondly, fabrication of an AM panel is being described. The AM panelwas fabricated with standard Complementary Metal-Oxide Semiconductor(CMOS) process on a (100) single crystal silicon wafer. After cleaning,well regions and body connections were deposited and patterned. Fieldoxidation was performed to define the active area of the transistorsusing silicon nitride as a hard mask. Then, a thin layer of thermaloxide was grown as gate oxide. After poly-Si deposition and gatepatterning, a source/drain region was formed by ion implantation withstandard self-alignment technology. Then, low temperature oxide (LTO)was deposited, and the wafer was annealed to densify the LTO and toactivate the implanted dopants simultaneously. After opening contactholes on the LTO layer, Al—Si alloy was deposited, and patterned forsource/drain electrodes and interconnections.

Thirdly, a flip-chip process of the AM panel and the LED micro-arraypanel is being described. After the CMOS process, a layer of PECVD SiO₂was deposited on the AM panel for passivation and holes were opened. ATiW/Cu (30/500 nm) seed layer was deposited by sputtering andphotoresist AZ4903 was coated and patterned by photolithography. A thickCu layer (8 μm) and solder layer (22 μm) were deposited by electricalplating. After reflow in the annealing furnace, excellent solder bumpswere formed in a ball shape, as is shown in FIG. 13. The LED micro-arraywafer was thinned and diced. After flipping the diced LED micro-arrayonto the AM panel, the device is completed as is shown in FIG. 10. Thecompleted device was packaged in a dual in-line package (DIP) 40 socketand electrically connected by wire bonding.

FIG. 8 shows a configuration of 8×8 LED driving circuits on an AM panelaccording to the present subject matter. The AM panel includes 8×8 pixeldriving circuits 201, a power source VDD 202, a ground 203, and inputsfor data signals 204 and inputs for select signals 205. The drivingcircuits 201 are selected from the group consisting of p-channel MetalOxide Semiconductor (PMOS) transistors; n-channel Metal OxideSemiconductors (NMOS) transistor; n-type amorphous silicon Thin FilmTransistors (n-type a-Si TFTs); p-type amorphous silicon Thin FilmTransistors (p-type a-Si TFTs); n-type poly crystalline silicon ThinFilm Transistors (n-type p-Si TFTs); p-type poly crystalline siliconThin Film Transistors (p-type p-Si TFTs); n-type SOI transistors; and/orp-type SOI transistors.

FIG. 9 is a schematic diagram of a pixel driving circuit according tothe present subject matter. Transistor T1 serves as a switchingtransistor and transistor T2 serves as a driving transistor. Whentransistor T1 is switched on by a scan signal, a data signal switchestransistor T2 on and is stored in capacitor C1. Then, transistor T2provides current to turn on the LED pixel whose p-electrode is connectedto the drain of transistor T2. Driving transistor T2 is designed with alarge W/L ratio to warrant enough current for the LED pixel.

FIG. 10 shows a microscopic image of an assembled AM LED micro-arraypanel. The LED micro-array 104 is thinned and polished, and then flippedon an AM panel 105. Light is emitted from the backside of the substrate.It is found that the aspect ratio of the AM LED micro-array could be ashigh as 100%, profiting from bottom emitting configuration.

FIG. 11 shows a typical current-voltage (I-V) characteristic of anindividual pixel in an AM LED micro-array panel. Since the LED and thedriving transistor are connected in series, the operating points aredetermined by the power supply voltage as well as the current-voltagecharacteristics of the LED and the driving transistor. From the I-Vcurve, it is shown that the AM panel has sufficient driving capabilityfor the LED micro-array.

FIG. 12A and FIG. 12B respectively show fully turned-on and individuallyturned-on images of an AM LED micro-array panel according to the presentsubject matter. The LED pixels have high brightness, good luminanceuniformity and individual controllability by the AM panel. FIG. 13 showsan AM panel on which a plurality of solder bumps are attached in rowsand columns.

According to the constitutions described above, individualcontrollability of each LED pixel in the LED micro-array as well asprevention of cross-talk between neighboring LED pixels is achieved. Inaddition, the present subject matter provides good luminance uniformityand high drive capability across a large area by driving each LED pixelwith an individual pixel circuit, as well as ensuring a small LED pixelpitch and high display resolution. Also, by this constitution,interconnection lines between an output of the AM panel and ap-electrode of the LED pixels is saved. The present subject matterovercomes the incompatibility between the LED process and the CMOSprocess.

Although the subject matter has been described with reference to theillustrated embodiment, the subject matter is not limited thereto.Rather, it will be obvious that the same may be varied in many ways.Such variations are not to be regarded as a departure from the spiritand scope of the subject matter, and all such modifications as would beobvious to one skilled in the art are intended to be included within thescope of the following claims.

What is claimed is:
 1. A method for manufacturing a monolithic LightEmitting Diode (LED) micro-display panel on an Active Matrix (AM) panel,comprising: providing a substrate of the LED micro-display panel;overlaying a plurality of layers of material on a surface of thesubstrate, the plurality of overlaying layers of material beingconfigured in combination to emit light when activated; patterning theplurality of overlaying layers of material by removing a part of eachoverlaying layers all the way down to the surface of the substrate;depositing a current spreading layer on the patterned plurality ofoverlaying layers of material and surface of the substrate; providing ametal multilayer on the current spreading layer; patterning the metalmultilayer in such a configuration that a first portion of the metalmultilayer, which lies on the patterned plurality of overlaying layersof material, and a second portion of the metal multilayer, which lies onthe surface of the substrate are conductively disconnected, therebyforming the monolithic LED micro-display panel; and combining themonolithic LED micro-display panel with the AM panel, said AM panelcomprising a plurality of active control circuit chips with conductivesolder material, said combining comprising flip-chip bonding themonolithic LEDs to the active control circuit chips via the conductivesolder material, wherein each of the monolithic LEDs is electricallyinsulated from one another and independently controllable bycorresponding ones of the active control circuit chips bonded thereto.2. The method according to claim 1, wherein the substrate of the LEDmicro-display panel comprises at least one material selected from thegroup consisting of: GaAs, SiC, Semi-insulating GaAs, Sapphire, andQuartz.
 3. The method according to claim 1, wherein the plurality ofoverlaying layers of material include a p-GaN layer, a Multiple QuantumWell (MQW) layer, and n-GaN layer.
 4. The method according to claim 1,wherein the current spreading layer forms p-electrodes of the LEDs, andthe metal multilayer forms n-electrodes of the LEDs and reflectivelayers on the p-electronics, and wherein the array of the multiple LEDsare arranged in rows, the method further comprising: connecting then-electrodes of LEDs in a same row to a common N-bus line that isconnected to a ground of the AM panel; and connecting the p-electrodesof the multiple LEDs individually to outputs of corresponding activecontrol circuit chips on which the multiple LEDs are flip-chip bonded.5. The method according to claim 1, wherein the patterning the pluralityof overlaying layers of material is performed by Inductively CoupledPlasma (ICP) etching using Plasma-Enhanced Chemical Vapor Deposition(PECVD) grown SiO2 masks.
 6. The method according to claim 1, whereineach of the multiple monolithic LEDs has a mesa structure with a sizeequal to or less than 300×300 μm2.
 7. The method according to claim 1,wherein the current spreading layer is a thin layer comprising at leastone layer selected from the group consisting of a Ni/Au layer and anAg/ITO layer.
 8. The method according to claim 1, wherein the metalmultilayer comprises a Ti/Al/Ti/Au layer.
 9. The method according toclaim 1, wherein the patterning of the plurality of overlaying layers ofmaterial by removing a part thereof all the way down to the surface ofthe substrate is performed by at least one method selected from thegroup consisting of wet etching and dry etching.
 10. The methodaccording to claim 1, further comprising: providing a passivation layeron the metal multilayer; and defining openings in the passivation layercontaining contact pads configured for the flip-chip bonding with theconductive solder material of the AM panel.
 11. The method according toclaim 10, wherein the contact pads contained in the openings defined inthe passivation layer comprises Ni/Au.
 12. The method according to claim10, wherein the passivation layer on the metal multilayer comprises atleast one selected from the group consisting of: SiO2, SiNx andphotoresist.
 13. A method for manufacturing an assembly of a monolithicLight Emitting Diode (LED) micro-display panel including a plurality ofLEDs thereon and an Active Matrix (AM) panel, comprising: providing asubstrate of the AM panel; providing active control circuits on thesubstrate of the AM panel; providing conductive solder material on theactive control circuits; providing a substrate of the LED micro-displaypanel comprising a patterned plurality layers of material overlaid on asurface of the substrate of the LED micro-display panel, and having acurrent spreading layer on the a patterned plurality of layers and on asurface of the substrate of the LED micro-display panel; providing ametal multilayer on the current spreading layer and patterning the metalmultilayer in a configuration that a first portion of the metalmultilayer, which lies on the patterned plurality of overlaying layersof material, and a second portion of the metal multilayer, which lies onthe surface of the substrate are conductively disconnected, form the LEDmicro-display panel; and combining the AM panel with the LEDmicro-display panel comprising monolithic LEDs electrically insulatedfrom one another, and flip-chip bonding the active control circuits tothe plurality of monolithic LEDs via the conductive solder material,with the monolithic LEDs independently controllable by the activecontrol circuits.
 14. The method according to claim 13, furthercomprising: depositing a passivation layer on the AM panel; definingholes on the passivation layer; depositing a seed layer by sputtering onthe passivation layer; coating a photoresist on the seed layer; andpatterning the photoresist by photolithography.
 15. The method accordingto claim 13, further comprising providing the active control circuits onthe substrate of the AM panel in a Complimentary Metal-OxideSemiconductor (CMOS) fabrication process.
 16. The method according toclaim 13, further comprising: thinning and dicing the LED micro-displaypanel; and flipping the diced LED micro-display panel onto the AM panelin such a configuration that each of the plurality of LEDs on the dicedLED micro-display panel faces corresponding one of the active controlcircuits on the AM panel.
 17. The method according to claim 13, furthercomprising forming the conductive solder material in a ball shape afterreflow in furnace.